And the Mountains Echoed question

I think he was under a lot of pressure by society, and also his Mother. Mother's are that way you know. He wasn't looking for children, so that mystical scar Nila had was probably from a hystorectomy. Perfect for Suleiman. The marriage got people out of his hair.

Apart from the societal pressure to marry which was most likely a contributing factor, Suleiman may have married Nila to selfishly keep Nabi in his employment.

Nabi was known to have relations with prostitutes, who would not be classified as "modest" by the standards of the society. So too would Nila not be classified as modest by these standards - evidence of which is given by her uncovered arms, her alcohol and drug consumption and the scornful eyes of the old conservative man in Shadbagh upon seeing her.
Suleiman may have married Nila because he knew she would eventually become a locus of Nabi's desire, which indeed happened instantly. As long as he could keep Nabi interested, he could keep him around. Why else would he let Nabi and his wife spend so much time alone, buying purses, visiting Shadbagh? He must have realized that Nabi and Nila were more than mere chauffeur and passenger when the opportunity of attaining custody of Nabi's niece, Pari, arose. He allowed their alone time and allowed Pari into his home, despite note wanting children, because the more tethers there were on Nabi, the more likely Nabi would stay, the more chances he would have to maybe, one day, muster the courage to profess his love.

He probably was under a lot of pressure from society (his mother at least) to marry and most certainly he couldn't publicly be gay. Marrying Nila was a smart move as neither of them would fit in a conventional marriage in Afghanistan. Both had their personal freedom (still limited in many ways) but they kept appearances as a married couple.

because he might not be accepted in the society if he was a gay, also being a gay in Islam is completely forbidden, so simply he married nila to sweep out any thoughts about him being gay , like a curtain to hide behind it

He was expected to marry. He had no choice. To come out as a gay man would have ruined him in society and in his family. Nila was different as well. Their marriage was an acceptable solution for both partners.

Yes his motivation to marry Nila must have rooted out of his need to shield himself from societal wrath he could have been subjected to if found to be a homosexual. If what Reham above mentioned is correct, he had all the more reason to never provide people any indication towards his sexual orientation.

I agree that he needed a social screen in the form of marriage ... but is it possible he chose Nila because she was beautiful (he was an artist) and impossible (starving his mother of the daughter-in-law she would have wanted)?

Social obligation !